1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method of the same. More specifically, the present invention relates to a thin film transistor which is provided with a semiconductor thin film exhibiting photosensitivity and is used under irradiation of light, a manufacturing method of the same, an active matrix substrate obtained by using those, and electronic equipment having those loaded thereon (e.g., a display and a photosensor).
2. Description of the Related Art
As a driving device for an active-matrix type liquid crystal display panel, it is widely known to form thin film transistors (referred to as “TFT” hereinafter) on an insulating substrate such as a glass substrate. Recently, because of multi-functionalization and micronization of semiconductor devices configured including TFTs, there has been a demand for achieving such a TFT that exhibits high durability and reliability. Further, in accordance with achievement in high-definition picture quality of a liquid crystal display and expansion in its general-purpose usage, there has been an increasing demand for improving the luminance of a backlight that is loaded on the liquid crystal display.
In general, an amorphous silicon film or a polycrystalline silicon film is used for TFT. With a semiconductor material such as the aforementioned silicon, electron-hole pairs are excited/generated by an optical energy under irradiation of light. The electron-hole pairs are observed as an increase in off-leak currents, when those pairs function as carriers especially when the TFTs are off. In such case, electric charges to be held in an electric-charge latch type element such as a liquid crystal display element are changed due to a leak current. This causes deterioration in the contrast and deterioration in the picture quality such as having a cross-talk image.
In order to cope with such issues, Japanese Unexamined Patent Publication 2004-302475 (FIG. 2B, and the like: Patent Document 1) discloses a technique which shields the light irradiated to an active layer of TFT by a film that has a light-shielding function.
FIG. 9 is a sectional view showing the TFT of Patent Document 1. Explanations will be provided hereinafter by referring to this drawing. In the explanation below, polycrystalline silicon is abbreviated to “poly-Si”, and amorphous silicon is abbreviated to “a-Si”.
A TFT 200 is formed on a glass substrate 207, and it is provided with an WSi film 221, an insulating film 223, an a-Si film 222, an insulating film 212, a poly-Si film 211, a gate SiO2 film 204, a gate Cr/n+ film 210, an interlayer SiO2 film 203, a wiring Ti/Al film 209, an SiN protective film 202, a flattening film 201, a pixel ITO (Indium Tin Oxide) film 208, and the like. Here, the structure from the WSi film 221 to the gate Cr/n+ film 210 is referred to as the TFT 200.
With this TFT, it is intended to block the light by providing a first light-shielding film, a first insulating film, a second light-shielding film, and a second insulating film underneath a semiconductor film in this order from the bottom side. That is, as shown in FIG. 9, the TFT includes, underneath the poly-Si film 211, the WSi film 221 that is one of the light-shielding films, the insulating film 223, the a-Si film 222 as the other light-shielding film, and the insulating film 212 from the bottom side. Each of the light-shielding film is patterned to have a larger area than that of the poly-Si film 211, and each has a function of reducing the amount of light reaching to the poly-Si film 211, which makes incident from the glass substrate 207 side.
Further, in a TFT depicted in Japanese Unexamined Patent Publication 9-213964 (FIG. 1B, and the like: Patent Document 2), a laminate configured with three layers containing a light-shielding film, an insulating film, and a semiconductor film receives patterning simultaneously. Note here that the light-shielding film is made of metal such as Cr, the insulating film is made of silicon oxide, and the semiconductor film is made of Poly-Si.
However, there is a following issue with the TFT that is disclosed in Patent Document 1. As described above, the first light-shielding film, the first insulating film, the second light-shielding film, and the second insulating film are provided underneath the semiconductor film. Thus, it is necessary to go through complicated manufacturing processes such as: forming and patterning the first light-shielding film (lithography and etching), forming the second insulating film, forming and patterning the second light-shielding film, and forming the second insulating film. That is, the manufacturing processes become complicated for obtaining highly light-resistant TFT, which results in increasing the manufacturing cost.
In the meantime, in the TFT depicted in Patent Document 2, the light-shielding film is made of metal such as Cr, whereas the insulating film and the semiconductor film are made of a silicon material. Thus, it is necessary to form the insulating film and the semiconductor film by plasma CVD after forming the light-shielding film by sputtering or vapor deposition. As described, two or more kinds of film-forming devices are used for forming a laminate of the TFT of Patent Document 2. Therefore, the manufacturing processes become complicated as well, thereby resulting in increasing the manufacturing cost. Moreover, each layer of the laminate receives patterning simultaneously, so that steps in the peripheral edge of the laminate become extremely larger compared to the case where each layer receives patterning individually. Therefore, step coverage of the gate insulating film for covering the steps of the peripheral edge of the laminate becomes very poor. Thereby, a gate leak current becomes easily generated.